The present invention relates to a portable telephone terminal and a power supply method that can effectively utilize electric energy of a power supply (battery) incorporated in the terminal.
As a result of recent progress of semiconductor fabrication processes, especially microfabrication techniques, processors and the like typifying a digital integrated circuit (IC) as a semiconductor device have been increasingly scaled down, and their minimum line width becomes narrower each year. With this trend for finer lines, operating voltage of the device itself is being lowered. Devices used in a digital system of a portable telephone terminal are no exception; portable telephone terminals employ microprocessors, signal processors and the like that operate on low voltage.
On the other hand, an analog system of a portable telephone terminal uses electronic devices typified by a transmission amplifier and an analog IC. In order to maximize performance of circuits in the analog system, the analog system effectively utilizes the maximum voltage of a battery incorporated in the telephone terminal. On the other hand, a device in a power supply system typified by a power-conserving control IC for optimally controlling power to each device is required to have an absolute withstand voltage such that the device can satisfactorily withstand the maximum voltage of the battery.
Thus, devices that operate on a voltage lower than the battery voltage and devices that operate using the battery voltage as it is are mixed in a portable telephone terminal. A so-called series power supply (constant-voltage power supply) is used as a source of low-voltage power supply to lower the battery voltage to a desired voltage level and supply the low voltage to specified circuits.
However, when the series power supply (series regulator) is used to supply the low voltage to devices of the portable telephone terminal, power loss occurs as the series power supply operates to supply the low voltage (constant voltage) because of characteristics of the series power supply. This indicates that the series power supply serving as the source of power supply consumes power that is intended to be supplied to IC and other devices. Thus, there is a problem in that electric energy possessed by the power supply (for example a lithium battery) is not effectively utilized.
In addition, there are not a few cases where the series power supply in the process of making the voltage constant consumes more power as heat than is consumed by the devices in the portable telephone terminal, depending on the conditions in which the series power supply is used.
Furthermore, as the portable telephone terminal itself is reduced in size, packaging density of parts used in the portable telephone terminal is increased. This makes it increasingly difficult to make thermal design to release heat generated by the series power supply to the outside of the portable telephone terminal.